In vehicles having internal combustion engine mounted thereon including automobiles, a technique of EGR (Exhaust Gas Recirculation) is sometimes employed for the purpose of NOx reduction in exhaust gas and improvement of fuel efficiency. This technique is a technology of taking out a part of exhaust gas exhausted from an internal combustion engine, and again breathing from intake side of the internal combustion engine, and has become widely used mainly in a diesel engine. However, in recent years, the technology is getting to be applied to a gasoline engine.
EGR system requires an apparatus to cool exhaust gas to a temperature at which the exhaust gas can be circulated. This is EGR cooler.
Even in a latent heat recovery equipment of a water heater, a heat exchanger is used for the purpose of CO2 reduction in combustion exhaust gas and reutilization of heat, and a secondary heat exchanger that can reduce a temperature of combustion gas exhausted at about 200° C. up to about 50˜80° C. is developed.
General structure of a heat exchanger recovering heat of exhaust gas is schematically shown in FIG. 1 and FIG. 2. Heat exchange section partitioned by two partition plates is provided at a part of exhaust gas flow passage constituted of an external cylinder, and the section is constituted of a heat exchanger which allows heat to escape in cooling water. Air pipe is connected to the partition plate at a perforated portion thereof, and exhaust gas flows in the air pipe in the heat exchange section. The cooling water flows around the air pipe. The air pipe includes a type constituted of a pipe merely made of a metal (FIG. 1) and a type having a fin provided in the pipe (FIG. 2).
Such a heat exchanger is constituted of metal members such as an external cylinder, a partition plate, an air pipe, and a fin in the air pipe, and those members are joined by brazing. Copper brazing filler, copper alloy brazing filler, nickel brazing filler, iron brazing filler and the like are used as a brazing filler metal. However, in EGR cooler and the like, because the exhaust gas temperature at the inlet side of the heat exchange section sometimes reaches up to about 800° C. and the temperature at the outlet side sometimes reaches up to about 200° C., nickel brazing filler (BNi-5, BNi-6 and the like of JIS Z3265) having excellent high temperature oxidation resistance and high temperature strength is frequently used.
The metal member constituting the heat exchanger requires the following characteristics.    (1) Brazability is good    (2) Corrosion resistance under usage environment is good. For example, in automobile applications, corrosion resistance to snow melting salt is good. Particularly, in EGR applications, corrosion resistance to LLC (Long Life Coolant; for example, ethylene glycol) is good. In water heater applications, corrosion resistance in the case of being exposed to outdoor environment is good.    (3) Corrosion resistance to cooling water (heat medium) is good. For example, in EGR applications, corrosion resistance to LLC (Long Life Coolant; for example, ethylene glycol) is good.    (4) Corrosion resistance to dew condensation of water is good. In a member exposed to engine exhaust gas and combustion exhaust gas, dew condensation is easy to occur in the vicinity of exhaust gas outlet side during operation, and dew condensation is easy to occur in exhaust gas contact portion after operation.    (5) High temperature strength and high temperature oxidation resistance are good. The reason for this is that a heat exchanger which recovers heat of exhaust gas is exposed to high temperature gas.